1. Field of the Invention
The present invention relates to a face detection method and apparatus for detecting a face image that includes a face from a digital image. The present invention also relates to a program for causing a computer to function as the face detection apparatus.
2. Description of the Related Art
Face detection techniques for detecting faces included in digital images have been studied in various fields, in particular, in the fields of image correction, security system, digital camera control, and the like, and different types of face detection methods are proposed. One such method is proposed as described, for example, in U.S. Patent Application Publication No. 20020102024, and non-patent literature “Fast Omni-Directional Face Detection”, Shihong LAO et al., MIRU 2004, pp. II271-II276, Jul. 2004. In the method, a sub-window on a digital image is scanned to sequentially determine whether the image displayed on the sub-window is an image that includes a face using a discriminator, and thereby the face included in the digital image is detected.
Generally, a face included in a digital image has unspecified inclination (rotational position of the face on the image), orientation (face orientation in the right and left bobbing direction), size, position, and the like, except for special cases, such as ID photos and the like. Therefore, it is often the case that a face included in a digital image is detected while varying the detection conditions, including the inclination, orientation, size, position, and the like of the face to be detected.
In the mean time, in the face detection method for detecting a face included in a digital image, a high face detection probability (fewer detection failures) with a short processing time is desirable.
In the methods in which the face is detected while varying the detection conditions, however, increased face detection probability (decreased detection failures) tends to longer processing time, since detection conditions need to be specified finely, while increased processing time tends to decreased detection probability (more detection failures), since the detection conditions need to be specified coarsely. That is, the face detection probability (extent of detection failures) is in the so-called trade-off relationship with the processing time. Therefore, it is difficult to satisfy both high detection probability (fewer detection failures) and short processing time. Consequently, it has been a challenge to increase the face detection probability while at the same time reducing the processing time as much as possible.
In view of the circumstances described above, it is an object of the present invention to provide a face detection method and apparatus capable of improving face detection probability with reduced processing time in the face detection process for detecting a face included in an image. It is a further object of the present invention to provide a program for causing a computer to function as the face detection apparatus.